1. Field of the Invention
The present invention relates to a thin film transistor substrate and a method for manufacturing the same, and more particularly, to a thin film transistor substrate in which plural types of thin film transistors are formed on an insulating substrate, the plural types of transistors differing in driving voltage, for example, a driver circuit, a power source booster circuit, and a level shift circuit.
2. Description of the Related Art
In recent years, by using polysilicon (poly-Si) thin film transistors (TFTs), pixel switch elements and also driver circuits can be formed on the same substrate in a liquid crystal device, organic EL display device, or the like which is formed on a low-cost glass substrate. A lower operation voltage is desired in the driver circuit for a lower power consumption. However, a voltage at some level or more is required in pixel operation. Therefore, there is required a technique in which plural types of TFTs with different operation voltage are mixedly fabricated on a substrate to form a driver circuit including a power source booster circuit, a level shift circuit, or the like.
Generally, to form TFTs having different operation voltages, a technique of changing a gate insulating film in thickness is adopted considering a withstand voltage. For example, Japanese published application H05-335573A (prior art 1) discloses a technique, as shown in FIG. 1, in which island-like active layers 302 are formed on an insulating substrate 301. Impurity doping regions 305a, 305b that are to be source/drain regions are formed in the active layers, a gate insulating film 303 is formed on the entire surface, and a gate electrode 304 of a peripheral circuit TFT 401 is formed on the gate insulating film. Then, a first interlayer insulating film 306 is formed on the entire surface, and a gate electrode 307 of a matrix circuit (switching) TFT 402 is formed. Thereafter, a second interlayer insulating film 308 is deposited, and metal wirings 309 are formed.
Japanese published application 2003-45892 (prior art 2) discloses a technique in which a low voltage driving TFT and a high voltage driving TFT are formed on an insulating substrate, according to a similar way to the technique disclosed in the prior art 1. In the low voltage driving TFT, a channel region between a source and drain regions does not overlap a gate electrode, whereas in the high voltage driving TFT the channel region overlap the gate electrode.
Further, Japanese published application H08-250742A (prior art 3) discloses a technique in which island-like active layers are formed on an insulating substrate, a first gate insulating film is formed selectively on the island-like active layer of a switching TFT. Then, a second gate insulating film is deposited on the entire surface, a gate electrode of the switching TFT and a gate electrode of a peripheral circuit TFT are formed simultaneously, and doping of impurity ions and an activation process with the use of laser light are performed.
However, the method disclosed in prior art 3 involves a problem in that an etching step of an insulating film which includes a photolithography step is additionally provided. Further, in a process of manufacturing a TFT, each of an impurity doping step and an activation step significantly depends on the thickness of the insulating film on the active layer. Thus, in the case where the respective steps are performed with insulating films (gate insulating films) that differ in thickness, it is difficult to achieve satisfactory circuit operation because it is difficult to match TFT characteristics, particularly, threshold voltages among plural kinds of TFTs.
In the impurity doping step, there arises a problem in that the number of photolithography steps and the number of impurity doping steps increase when impurities are doped in a self-aligning manner as to all the plural kinds of TFTs. Further, when the thickness of the insulating film on the active layer exceeds 120 nm, there also arises a problem in that phosphorous is difficult to be doped because phosphorous is low insulating-film transmission capability. In order to avoid the above-mentioned problems, impurity may be doped simultaneously with respect to the plural kinds of TFTs immediately after the formation of the active layers, as disclosed in, for example, the prior art 1. However, in the case where the impurity is not doped in a self-aligning manner, an overlap or offset structure is invited by gate alignment, which leads to variation in threshold voltage. Further, the overlap structure causes an increase of a parasitic capacitance, and the offset structure causes lowering TFT ON current. Both of the structures invite lowering a circuit operation frequency.
In the activation step, a general thermal activation method with the use of a furnace requires a processing time of 10 hours or more, and provides a low throughput. Thus, an activation method with photoirradiation such as a laser activation method or a lamp heating method or a rapid heating method in which a gas heated at approximately 600° C. or more is sprayed, is used as an activation method with a high throughput. However, the above activation method with a high throughput involves large thermal shock, and thus, this provides a problem in that a gate electrode is likely to be peeled off. Further, a cooling effect on the active layer by the insulating film directly thereabove is extremely large under the influence of thermal diffusion compared with the thermal activation method. This causes a problem in that thermal hysteresis is changed depending on the insulating film thickness, which leads to change a maximum elevated temperature of the active layer. Thus, besides a problem by varying the activation rates, particularly in a low-temperature poly-Si TFT on a glass substrate manufactured at approximately 600° C. or less, secondary effects in the activation step, such as a decrease in defect density of a poly-Si film and a densification of the insulating film, are influenced and varied. Moreover, in the laser activation method, there arises a problem in that a light reflectance changes depending on the insulating film thickness. That is, in the prior art 3, when the insulating films on the active layer differ in thickness in the activation step, a difference develops in the activation rate, thereby causing a channel region and an impurity doping region in the active layer to be formed with different sheet resistances. As a result, there are formed TFTs differing in mobility, threshold voltage, reliability, and the like.
The method can be adopted, in which impurity doping and activation are conducted before the formation of an insulating film as disclosed in the prior art 1, in order to avoid change in layer resistance of the channel region and the impurity doping region depending upon the insulating film thickness. However, in this case, the above-described secondary effects in the activation step cannot be obtained.